This invention relates to low-loss optical fiber wave guides and more particularly to a system for nondestructively determining the optical transmission characteristics of such optical wave guides.
The use of low-loss fiber optic wave guides to transmit intelligence utilizing light as the carrier is now well known. The number of particular systems, methods, arrangements, schemes, etc., for utilizing the transmission of light through fiber optic wave guides for the transmission of intelligence is, indeed, very great.
In many of these particular applications of fiber optic wave guides, it is necessary that the transmission characteristics of the wave guide be known. It has been found that the transmission and dispersion characteristics vary quite greatly among wave guides from different manufacturers and even among the wave guides supplied by a single manufacturer. The composition of the glass and impurities and inhomogeneities therein appear to be the main factors affecting these characteristics. Moreover, it is usual that a manufacturer will supply transmission data for its wave guides at a single wavelength of light. That data is of limited practical value if the wave guide is to be used to transmit at a different wavelength since the optical attenuation coefficient, .alpha.(.lambda.), is strongly dependent upon the wavelength of the light transmitted. In a typical low-loss fiber, .alpha.(.lambda.) can range from 4 dB/km at 800 nm to over 100 dB/km at 400 nm.
One example of an application where this is of critical importance is in connection with the underground testing of nuclear explosives. It is common practice in such testing to emplace the explosive to be tested at the bottom of a hole drilled into the earth in order that the radioactive materials produced by the explosion be contained within the earth. Radiation produced upon detonation of the nuclear explosion, such as neutrons or gamma rays, is utilized to provide information on the performance of the nuclear device. A fluor is placed in the path of the radiation and the light produced by the action of the radiation on the fluor is transmitted from the underground detonation site uphole to the earth's surface through fiber optic wave guides. A knowledge of the transmission characteristics of the light guide at the wavelengths emitted by the fluor is, of course, essential to the unfolding of intelligence on the radiation seen by the fluor.
A common technique for measuring .alpha.(.lambda.) of fiber optic wave guides utilizes a high-intensity arc lamp in conjunction with an external wavelength dispersion device, such as a monochromator or interference filter, and a photoelectric detector. In practice, the attenuation is measured in two different lengths of fiber and the difference is used to obtain the attenuation per unit length of the fiber. It will be appreciated that this procedure necessitates a breaking of the fiber to get the two lengths thereof.